The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with multiple quadrature volume coils for medical diagnostic applications of magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated, that the present invention will also find application in other multiple coil techniques, spectroscopy, phased array coil techniques, imaging for other than medical diagnostic purposes, and the like.
Conventionally, magnetic resonance imaging systems generate a strong, uniform static magnetic field in a free space or bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object in the bore to be imaged. The polarized object then possess a macroscopic magnetic moment vector pointing in the direction of the main magnetic field. In a superconducting main magnet assembly, annular magnets generate the static magnetic field B.sub.0, along a longitudinal or z-axis of the cylindrical bore.
To generate a magnetic resonance signal, the polarized spin system is excited by applying a radio frequency field B.sub.1, perpendicular to the z-axis. Typically, a radio frequency coil for generating the radio frequency field is mounted inside the bore surrounding the sample or patient. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis back toward alignment, the precessing magnetic moment generates a magnetic resonance signal which is received by the radio frequency coil in a reception mode.
For imaging, a magnetic field gradient coil is pulsed for spatially encoding the magnetization of the sample. Typically, the gradient magnetic field pulses include gradient pulses pointing in the z-direction but changing in magnitude linearly in the x, y, and z-directions, generally denoted G.sub.x, G.sub.y, and G.sub.z, respectively. The gradient magnetic fields are typically produced by a gradient coil which is located inside the bore of the magnet and outside of the radio frequency coil.
Conventionally, when imaging the torso, a whole body radio frequency coil is used in both transmit and receive modes. By distinction, when imaging the head, neck, shoulders, or an extremity, the whole body coil is often used in the transmission mode to generate the uniform excitation field B.sub.1 and a local coil is used in the receive mode. Placing the local coil close to the imaged region improves the signal-to-noise ratio and the resolution. In some procedures, local coils are used for both transmission and reception. One drawback to local coils it that they tended to be relatively small. The whole body coils are typically used for elongated regions, such as the spine. One technique for adapting surface coils for imaging an elongated region is illustrated in U.S. Pat. No. 4,825,162 of Roemer, in which a series of surface coils are lapped to construct a phased array.
Other radio frequency coil designs include a multi-modal coil known as the "birdcage" coil. See, for example, U.S. Pat. No. 4,692,705 of Hayes. Typically, a birdcage coil has a pair of end rings which are bridged by a plurality of straight segments or legs. In a primary mode, currents in the rings and legs are sinusoidally distributed which results in improved homogeneity along the axis of the coil. Homogeneity along the axis perpendicular to the coil axis can be improved to a certain extent by increasing the number of legs in the coil. Typically, a symmetric birdcage coil has eight-fold symmetry. Such a symmetric birdcage coil with N legs exhibits N/2 mode pairs. The first (N/2)-1 mode pairs are degenerate, while the last mode pair is non-degenerate. The primary mode of such an eight-fold symmetric birdcage coil has two linear modes which are orthogonal to each other. The signals from these two orthogonal or quadrature modes, when combined, provide an increased signal-to-noise on the order of 40%. The simplest driven current pattern or standing waves defined by superpositions of degenerate eigenfunctions. For a low-pass birdcage of n meshes driven at is lowest non-zero eigenfrequency, the current in the n-th mesh is given by sin(2.pi.n/N+.phi.). The phase angle .phi. determines the polarization plane of the resulting B.sub.1 radio frequency field and can be varied continuously by suitable application of driving voltages. The alignment and isolation of the two linear modes of a birdcage coil are susceptible to sample geometry. That is, the sample dominates the mode alignment and isolation between the two linear modes.
Birdcage coils, like other magnetic field coils, undergo mutual inductive coupling when positioned adjacent each other. As the coils approach each other, the mutual inductive coupling tends to increase until a "critical overlap" is reached. At the critical overlap, the mutual inductance drops to a minimum. As the coils are moved towards a complete coincidence from the critical overlap, the mutual inductive coupling again increases. See, "Optimized Birdcage Resonators For Simultaneous MRI of the Head and Neck" Leussler Society of Magnetic Resonance in Medicine Abstracts, page 1349, 1993.
Although the critical overlap reduces the mutual coupling between birdcage coils, the mutual coupling is dramatically changed when the sample changes. Introducing a different geometry sample in the two lapped birdcage coils alters the alignment of their modes. The mode isolation in the coils changes, which in turn, affects the symmetry and therefore the mutual coupling between the coils. The greater the mutual coupling, the larger the noise correlation between coils and the lower the combined signal-to-noise ratio. Further, electrical optimization of such lapped birdcage coils is very complex. The isolation process between coils is iterative and time-consuming. That is, the linear modes in each birdcage coil are aligned to their respective coupling points on the coil and isolated with respect to one another, as well as from the two linear modes of the other coil.
The present invention contemplates a new and improved radio frequency coil design which overcomes the above-referenced problems and others.